1. Field of the Invention
The present invention relates to proportional control type remote-control direction switching control valve devices in which the operational mode of a direction switching control valve is established by a hydraulic feedback circuit, and an opening of a valve passage is adjusted to control the speed of a main hydraulic actuator.
2. Description of the Prior Art
Most cranes employ a hydraulic actuator to extend or contract a boom. Generally, the hydraulic actuator includes a direction switching control valve connected between the actuator and a hydraulic power supply unit and operable to control both the direction and speed of boom movement.
Such a direction switching control valve may be operated directly by a handle. However, in order to remotely operate the valve, a sub-hydraulic actuator can be connected to the direction switching control valve and used to control the hydraulic circuit.
In the past, remotely-controlled direction switching control valve devices of the type as shown in FIG. 1 have been proposed. As illustrated, a 6-port 3-position direction switching control valve 2 has a spool which is switched between valve positions a, b and c to vary the opening of valve passages. The spool is normally held at the neutral position by means of return springs 3 and 4 and can be operated either directly by a hand lever 1 or driven by a sub-hydraulic actuator 8 connected by a rod 6. A cylinder 7 has liquid chambers 10 and 11 defined by a piston 9, the liquid chamber 10 being connected between solenoid valves 15 and 16 and the liquid chamber 11 being connected between solenoid valves 16 and 17.
The solenoid valves 15, 16 and 17 are connected in series between a hydraulic power source P and a tank R. Normally in a position e which closes a circuit, the solenoid valve 15 is switched to a position d against a spring 18a when an electromagnetic coil 18 is excited. The solenoid valves 16 and 17 are normally in positions g and k, respectively, and when electromagnetic coils 19 and 20 are excited, the valves 16 and 17 are switched to closed positions f or h, respectively.
A control circuit 13 compares an electric signal from a detector 14, such as a differential transformer or a potentiometer which detects the displacement of the piston 9, and an electric signal from a circuit 12 being a differential transformer or a potentiometer actuated by an operator. The electric control circuit 13 generates an output signal that energizes the electromagnetic coils 18, 19 and 20 to control the hydraulic actuator 8. When the position of the piston 9, as measured by the detector 14 is in coincidence with the predetermined target value of the operating circuit 12, the output signal from the control circuit 13 is terminated to stop the operation of the hydraulic actuator 8. For example, when rightward movement of hydraulic actuator 8 is desired, the solenoid valves 15 and 16 are energized; when leftward movement is desired, the solenoid valves 15 and 17 are energized; and when stoppage of the hydraulic actuator 8 is desired, the solenoid valves 16 and 17 are energized.
One disadvantage of the prior art direction switching control valve device described above exists when each of the electromagnetic coils 18, 19 amd 20 is de-energized. Under those conditions, variations in the pressure of the tank R can produce undesirable movement of the piston 9. When the pressure of the tank R increases, the piston 9 moves leftward because the pressure-receiving area on the left side of the piston 9 is less than the pressure-receiving area on the right side thereof. Another disadvantage is that an element similar to the rod 6 cannot be utilized on the right side of the piston 9.